1. Field of the Invention
This invention relates to an electric tool and particularly to a torque control device such as an electric screw-driver.
2. Description of the Prior Art
Electric torque control devices incorporating a transformer are known in the prior art. However, these devices present a disadvantage in that, the torque is set in response to a value of a power source voltage.
The variation of the voltage influences and the number of revolution of the motor, resulting in a delay in the time during which the screw-in operation of machine screws is completed in an assembly line.
There is also known an electric torque control device incorporating transistors in which the electric current is terminated as soon as it reaches a predetermined value. In this type of device, the heat which is produced by a motor which is in operation for a long period increases a demagnetization, resulting in the fluctuation of the electric current. Consequently, the value of the current increases and, accordingly, a slight change occurs in the relationship between the predetermined value of the current and the torque. Further, it is impossible to exactly detect the torque since a time during which the detection is effected is not constant when the transistor detects a predetermined torque, for example current. Moreover, the primary voltage is not constant and the fluctuation of the primary voltage influences the secondary voltage, resulting in the fluctuation of the torque.